In the human digestive system, food is mechanically ground up and chemically broken down into small molecules that can be used by the body. Digestive enzymes from salivary, gastric, pancreatic, and intestinal glands hydrolyze proteins to amino acids, starch to simple sugars, and fats to glycerol and fatty acids. The digestive system consists essentially of the mouth, esophagus, stomach, small intestine and large intestine. The stomach acts as a reservoir which releases food slowly into the small intestine. Digestion is completed in the small intestine where the food is absorbed into the bloodstream or lymph vessels. The small intestine is around 5 meters in length and it normally takes food around 4-5 hours to pass through it before reaching the large intestine.
In the human diet, carbohydrates carry energy and supply carbon atoms to biosynthetic pathways. They are found in nature as structural elements in most types of cells and tissues. Based on the number of monomeric units which they contain, carbohydrates are classified as monosaccharides, disaccharides, oligosaccharides (chains of 3-10 monosaccharide units), or polysaccharides. Starch is one of the most common naturally occurring polysaccharides. The monomeric unit of starch is glucose. Other common polysaccharides are glycogen and cellulose. These are respectively the carbohydrate used to store energy by animals and the substance forming the skeleton of plant structures. The monomeric unit of both of these polysaccharides is also glucose.
Starch is present within plant cells as discrete microscopic granules enclosed in phospholipid membranes. The size and form of a granule is often characteristic of the plant of its origin. Within each granule, amylopectin forms a branched, helical, crystalline system with the linear amylose dispersed within the amylopectin structure. The digestibility of raw starch depends upon the accessibility and crystalline structure of its granules. The granules are gelatinized during cooking and freshly cooked starchy foods are easily digestible.
In western countries, starch constitutes about 60% of carbohydrates consumed. Starch is hydrolyzed in the intestinal tract into oligo-, di- and mono-saccharides. The enzymatic breakdown of polysaccharides, such as starch, into oligosaccharides actually starts in the saliva. However, the bulk of starch breakdown occurs in the small intestine. Here, enzymatic splitting takes place of poly- and oligosaccharides into low-molecular, and hence absorbable, monosaccharides such as glucose. Starch breakdown in the small intestine can be a quite rapid process so that the majority of the starch is broken down within one hour of entering the small intestine from the stomach.
The total starch (TS) content of a food is measured as the yield of glucose from a finely milled or homogenized sample of the food in which the starch is completely gelatinized at 100.degree. C., treated with potassium hydroxide to ensure complete dispersion of the starches to an amorphous, digestible form and then enzymatically digested with pancreatin and amyloglucosidase.
Starches can be classified into a number of different types or fractions according to their behavior when incubated with enzymes without prior exposure to dispersing agents.
Three types of starch can be identified according to how they react to controlled periods of enzymatic digestion of homogenized and non-homogenized food samples. Firstly, rapidly digestible starch (RDS) consists mainly of amorphous and dispersed starch. It is typically found in high amounts in starchy foods which have been cooked by moist heat such as bread and potatoes. It is measured chemically as the starch which is converted to the constituent glucose molecules within 20 minutes of enzyme digestion. The next type is slowly digestible starch (SDS), which like RDS is in normal circumstances completely digested in the small intestine. However, its digestion proceeds more slowly than RDS. This fraction includes for example physically inaccessible amorphous starch. It is measured chemically as the starch converted to glucose after a further 100 minutes of enzyme digestion. The final starch type is resistant starch (RS) which may potentially resist digestion in the small intestine. It is measured chemically as the difference between TS obtained from the homogenized and chemically treated sample and the sum of RDS and SDS generated from non-homogenized food samples by enzyme digestion.
RS can be further subdivided into three sub-types. These are RS.sub.1 which is starch resistant to digestion because it is in a physically inaccessible form such as partly milled grains and seeds; RS.sub.2 which has a granular form which is particularly resistant to enzyme digestion; and RS.sub.3 which is the starch fraction most resistant to digestion and which mainly consists of retrograded amylose formed during the cooling of gelatinized starch.
RS which is not digested in the small intestine passes through to the large intestine where it is fermented by colonic bacteria to produce short chain fatty acids, carbon dioxide and methane. It is evident that the speed at which a starchy food is digested will depend upon the relative proportion which it contains of RDS, SDS and the different RS sub-types.
The amounts and relative amounts of RDS, SDS and RS in foods are highly variable. They depend partly on the source of starch, but also on the type and extent of processing which the food is subjected to. It should be noted that, for most starchy foods, cooking converts starch to a readily digestible form. The Table below provides data for the in vitro digestibility of starch in a variety of foods. The values are obtained by carrying out assays in accordance with the techniques described above.
______________________________________ % RDS % SDS RS.sub.1 RS.sub.2 RS.sub.3 ______________________________________ Flour, white 38 59 -- 3 t Shortbread 56 43 -- -- t Bread, white 94 4 -- -- 2 Bread, wholemeal 90 8 -- -- 2 Spaghetti, white 55 36 8 -- 1 Biscuits, made with 50% 34 27 -- 38 t raw banana flour Biscuits, made with 50% 36 29 -- 35 t raw potato flour Peas, chick, canned 56 24 5 -- 14 Beans, dried, freshly cooked 37 45 11 t 6 Bean, red kidney, canned 60 25 -- -- 15 ______________________________________
In the above Table, values are expressed as a percentage of the total starch present in the food. The index "t" represents that only a trace of the particular starch type could be detected. It is evident from the data set out in the above Table that different foods have quite different relative contents of the different starch types.
When starch is digested in the small intestine it causes the level of glucose present in the blood to rise. The level of blood glucose in a healthy individual usually lies in the range 75-125 mg/100 ml. A greater glucose level can lead to heart, circulation, eye and kidney problems. A lower glucose level can lead to fatigue, fainting and hypoglycemic shock.
The highest point of the blood glucose level engendered by the indicated digestion of a food is known as the "glycemic response" of the food. The precise glycemic response of a food will vary according to the amount of food eaten and from individual to individual, depending upon such factors as the properties of the food, the efficiency of the individuals' digestive systems, and whether they suffer from diabetes.
With respect to digestion and health, it is important--as previously noted--for the blood glucose level of an individual to be controlled within a certain range. The two factors of greatest importance to enable this control are the activity of the liver and the balance of hormones. The liver has a certain autonomy. It is presently understood to have sensors which monitor blood glucose level. When the level is high, the liver removes glucose from the blood; when the level is low, it releases glucose into the blood.
In the hormonal regulation of blood glucose, the balance between insulin and glucagon is of prime importance. These two hormones are secreted by the pancreas in varying ratios, depending primarily upon the prevailing concentration of blood glucose. Any increase in the blood glucose level stimulates increased secretion of insulin, whereas a decrease in the blood glucose level stimulates the secretion of glucagon.
Some individuals are unable to control their blood glucose level by their own naturally produced insulin. This leads to the disease of diabetes. This disease is characterized by an above normal concentration of glucose in the blood. Diabetes is presently the third leading cause of death in the United States, where it kills 300,000 people per year. In 1950, there were 1.2 million diabetics in the United States, in 1975 there were 5 million diabetics and in 1991 there were more than 11 million diabetics. Accordingly, it is increasingly important to provide effective palliatives for the disease.
There are two types of diabetes which have different underlying causes. In Type 1 or insulin-dependent diabetes, there is an absolute deficiency of insulin and the patient may require regular injections of insulin to maintain glycemic control. In addition to insulin, diet and exercise must be carefully regulated in order to maintain good blood-sugar control.
In contrast, in Type 2 or non-insulin dependent diabetes, the pancreas is producing insulin, although it may not be doing so at normal levels. Although insulin is present, blood glucose levels are still abnormal because the body does not respond to it. However, the cause of this insulin resistance is presently unknown. Of people who are diagnosed as having this form of diabetes, 80% are overweight. Some non-insulin dependent diabetics can control their condition through diet and exercise alone. Other diabetics may need a combination of diet, exercise and medications. Medications for this type of diabetes include a class of drugs called oral hypoglycemic agents that help non-insulin dependent diabetics use blood glucose better.
A related disease, hypoglycemia, is caused by excessive circulating insulin. This is normally a result of an accidental overdose of insulin by a diabetic. This excess insulin results in a lowering of the blood glucose level. This particularly affects the brain which utilizes glucose as its main source of energy. Mild hypoglycemia can result in a lack of coordination. If the insulin excess is great enough, convulsions may occur, followed by coma.
With regard to the characteristics of the food itself, carbohydrates are an important part of a diabetic's diet. It is evident that diabetics should usually avoid foods having a high glycemic response, i.e., those which result in a relatively high level of blood glucose soon after digestion. Instead, diabetics require foods having a relatively low glycemic response which produce a slower rate of glucose release into the blood. Slowing the rate of release of glucose into the blood reduces the risk of both hyperglycemia and hypoglycemia.
This slowing of the glucose release rate can be achieved by reducing the rate of digestion of the food. Thus, if the speed of digestion can be lowered, the initial rush of glucose from RDS can be prevented and instead replaced by a controlled and lengthier digestive process along the whole length of the small intestine leading to slow release of glucose into the blood stream. Foods which only slowly release glucose into the blood stream may also be advantageous in suppressing the appetite of an individual, and for assisting an individual to perform sustained physical activity. In the former case, the presence of an above normal level of glucose in an individual's bloodstream may cause a feeling of satiety and so discourages snacking. In the latter case, the continual release of glucose into the bloodstream assists an individual to perform strenuous exercise as the glucose acts as a readily accessible form of fuel to working muscles.
Particularly as to the effect of a food's properties upon the glucose release rate, there are a number of basic principles concerning the glycemic response of a food. For instance, a food which includes a substantial content of RDS will possess a higher glycemic response, weight for weight, than a food formed substantially from SDS. In turn, such SDS-containing foods will give rise to a greater glycemic response than foods formed mostly from RS. In general, slowly digested foods produce flatter glycemic responses. Such foods have been termed "lente carbohydrate foods."
Several other characteristics of a food can affect its glycemic response. These include its particle size and texture, and any disruption which has taken place of its cell wall structure. Thus, cooked rice grains give rise to a smaller glycemic response than cooked flour. Cracking or milling of cereal grains to produce progressively greater disruption and finer particle sizes result in increased glycemic response values. Foods of coarse consistency possess smaller glycemic response values when swallowed without chewing, reinforcing the importance of particle size.
Further with respect to effecting modifications in foods' physical properties, the encasing of foods in different coatings is known. However, the various embodiments as taught in the art are characterized by disadvantages.
For instance, U.S. Pat. Nos. 2,517,595 and 2,611,708 disclose food articles encased in calcium-alkali and calcium pectinate and pectate films. Both identify rice as one of the foods that can be thusly coated.
These references indicate that the coating offers resistance against dirt and bacteria. They also state that food products provided with the coating can be eaten with the coating left on. Yet further, both references teach that if the coated food is cooked before eating, such as by boiling in water, then the film will dissolve in the cooking water and thereby be entirely removed.
U.S. Pat. No. 2,611,708 still additionally indicates, in this regard, that the indicated dissolution and entire removal is effected where the calcium content of the film is low. This patent in particular teaches that the food article is first coated with pectinate or pectate; then contacting with a calcium salt solution is effected while this coating is wet, to convert the alkali pectinate or pectate into a calcium-alkali or calcium pectinate or pectate. It further teaches that if the replacement of alkali ions is virtually complete, the resulting calcium pectinate or pectate films are stronger and more suited for the coating of foods which are to be cooked; in such instance, the film is softened during cooking, and can be easily stripped off the food article--if desired, by immersing the coated food article in hot water containing a small amount of a calcium sequestering agent.
These references do not disclose or suggest a food product including both a cooked and hydrated carbohydrate core, and a cation-crosslinked polysaccharide coating to substantially reduce the core's glycemic response; glycemic response is not mentioned. These references also do not disclose or suggest a food product with a carbohydrate core and such a glycemic response-reducing, cation-crosslinked polysaccharide coating which is disclosed to be insoluble in boiling water.
Yet further, cooking is discussed only in the context of occurring after crosslinking of the polysaccharide. There is no teaching or suggestion of effecting the crosslinking during cooking in an aqueous medium, or of a product obtained by such means.
U.S. Pat. No. 2,703,286 discloses the coating of foodstuffs with a combination of low methoxyl pectinate modified by a calcium salt, and methyl cellulose. A calcium chloride solution is applied to the foodstuff before and/or after application of the methyl cellulose and pectinate coating composition; if completely demethoxylated sodium pectinate is employed, the methyl cellulose is not required. Foodstuffs disclosed as being suitable include fruit bars, candy bars, cereal bars prepared by compressing mixtures of ingredients including cooked flour, and dried meat bars.
Here also, there is no disclosure or suggestion of a food product including both a cooked and hydrated carbohydrate core, and a cation-crosslinked polysaccharide coating which substantially reduces the core's glycemic response; once again, glycemic response is not mentioned. Further, there is no disclosure or suggestion of a food product with a carbohydrate core and such a glycemic response-reducing, cation-crosslinked polysaccharide coating which is insoluble in boiling water.
Yet additionally, in this reference cooking is discussed only in the context of prior treatment for starting materials, with the application of the pectinate coating and the crosslinking being subsequently effected. As with U.S. Pat. Nos. 2,517,595 and 2,611,708, there is no teaching or suggestion of effecting the crosslinking during cooking in an aqueous medium, or of a product obtained by such means.
U.S. Pat. No. 5,360,614 discloses encapsulating metabolizable carbohydrate in a time delay release layer, to provide a controlled release upon ingestion, and modulate the blood glucose response. This delayed release action is indicated to be helpful in counteracting the effects of diabetes; further administration of thusly coated carbohydrates is stated to be useful in conjunction with exercise programs calling for sustained effort. Among the coatings disclosed in this reference is ethyl cellulose. Controlled release of carbohydrates from pre-gelatinized starch granules obtained from cereals, such as rice, is also disclosed.
In this reference, there is no teaching or suggestion of crosslinking the coating at all.
European Application No. 487,340 discloses fried food compositions, encased in cation crosslinked polysaccharide coatings to impede the penetration of oil during the frying process; the products thusly coated and fried have a low concentration of the oil. Particularly, Example 31 teaches the soaking of potato strips in an aqueous solution of low molecular weight pectin and calcium for 6 minutes at 85.degree. C., with the thusly treated strips being subsequently fried in oil.
With regard to the present invention, this reference does not disclose or suggest a crosslinked polysaccharide-coated food which is ready to eat in the nonfried condition; rather, the food requires frying before consumption. The unfavorable properties which characterize fried foods are well known in the art; this point is of course recognized in European Application No. 487,340 itself, with the very purpose of the invention set forth therein being to reduce the disadvantageous effects of frying. In any event, fried foods are further known as being particularly unsuitable for individuals attempting to minimize their caloric intake, or who are afflicted with conditions affecting the body's ability to maintain a proper blood glucose level--e.g., diabetes and hypoglycemia, as discussed herein.